Thermal dissipation of the quantum spin Hall edge states in HgTe/CdTe quantum well.

Quantum spin Hall effect is characterized by topologically protected helical edge states. Here we study the thermal dissipation of helical edge states by considering two types of dissipation sources. The results show that the helical edge states are dissipationless for normal dissipation sources with or without Rashba spin-orbit coupling in the system, but they are dissipative for spin dissipation sources. Further studies on the energy distribution show that electrons with spin-up and spin-down are both in their own equilibrium without dissipation sources. Spin dissipation sources can couple the two subsystems together to induce voltage drop and non-equilibrium distribution, leading to thermal dissipation, while normal dissipation sources cannot. With the increase of thermal dissipation, the subsystems of electrons with spin-up and spin-down evolve from non-equilibrium finally to mutual equilibrium. In addition, the effects of disorder on thermal dissipation are also discussed. Our work provides clues to reduce thermal dissipation in the quantum spin Hall systems.

Species richness patterns and the determinants of larch forests in China.

Larch forests are important for species diversity, as well as soil and water conservation in mountain regions. In this study, we determined large-scale patterns of species richness in larch forests and identified the factors that drive these patterns. We found that larch forest species richness was high in southern China and low in northern China, and that patterns of species richness along an elevational gradient depend on larch forest type. In addition, we found that patterns of species richness in larch forests are best explained by contemporary climatic factors. Specifically, mean annual temperature and annual potential evapotranspiration were the most important factors for species richness of tree and shrub layers, while mean temperature of the coldest quarter and anomaly of annual precipitation from the Last Glacial Maximum to the present were the most important for that of herb layer and the whole community. Community structural factors, especially stand density, are also associated with the species richness of larch forests. Our findings that species richness in China's larch forests is mainly affected by energy availability and cold conditions support the ambient energy hypothesis and the freezing tolerance hypothesis.

Kinetics and mechanisms of pH-dependent degradation of halonitromethanes by UV photolysis.

Halonitromethanes (HNMs) are one of the most toxic groups of disinfection by-products. The pH-dependent degradation kinetics and pathways of four HNMs, namely bromonitromethane (BNM), dichloronitromethane (DCNM), dibromonitromethane (DBNM) and trichloronitromethane (TCNM), by ultraviolet (UV) photolysis at 254 nm were studied at pH 3-9. The UV photolysis in a dilute aqueous solution followed first-order kinetics. The photolysis rates of all four HNMs were low at pH 3-5, while that of TCNM was low at all pHs tested. Nevertheless, the photolysis rates of BNM, DCNM and DBNM increased with increasing pH, showing sharp increases as the pH neared their pK(a) values. The increases were correlated with their pH-dependent molar absorptivities, which were determined by the sizes of their deprotonated fractions. Homolysis was likely to be the major photolysis pathway for all four HNMs to produce halides, nitrite and nitrate at acidic pHs when the HNMs were not deprotonated. At high pHs, however, the conjugation systems of the deprotonated mono- and di-HNMs made heterolysis possibly the dominant pathway for the formation of carbon dioxide, nitrite and halides as major products for di-HNMs, and the formation of nitrite, halides and other unknown organics for mono-HNMs. The UV energy required for a 50% degradation of deprotonated HNMs in the real water sample was similar to that needed in UV disinfection processes, suggesting the effectiveness of UV photolysis in controlling HNMs that form conjugation systems at neutral to alkaline pHs.

Bromate formation from bromide oxidation by the UV/persulfate process.

Bromate formation from bromide oxidation by the UV/persulfate process was investigated, along with changes in pH, persulfate dosages, and bromide concentrations in ultrapure water and in bromide-spiked real water. In general, the bromate formation increased with increasing persulfate dosage and bromide concentration. The bromate formation was initiated and primarily driven by sulfate radicals (SO(4)(•-)) and involved the formation of hypobromous acid/hypobromite (HOBr/OBr(-)) as an intermediate and bromate as the final product. Under the test conditions, the rate of the first step driven by SO(4)(•-) is slower than that of the second step. Direct UV photolysis of HOBr/OBr(-) to form bromate and the photolysis of bromate are insignificant. The bromate formation was similar for pH 4-7 but decreased over 90% with increasing pH from 7 to above 9. Less bromate was formed in the real water sample than in ultrapure water, which was primarily attributable to the presence of natural organic matter that reacts with bromine atoms, HOBr/OBr(-) and SO(4)(•-). The extent of bromate formation and degradation of micropollutants are nevertheless coupled processes unless intermediate bromine species are consumed by NOM in real water.

Influence of pH on the formation of sulfate and hydroxyl radicals in the UV/peroxymonosulfate system.

The influence of pH on the degradation of refractory organics (benzoic acid, BA) in UV(254 nm)/Peroxymonosulfate (UV/PMS) system was investigated. The degradation of BA was significantly enhanced at the pH range of 8-11, which could not be explained only by the generally accepted theory that SO(4)(•-) was converted to HO(•) at higher pH. A hypothesis was proposed that the rate of PMS photolysis into HO(•) and SO(4)(•-) increased with pH. The hypothesis was evidenced by the measured increase of apparent-molar absorption coefficient of PMS (ε(PMS), 13.8-149.5 M(-1)·cm(-1)) and photolysis rate of PMS with pH, and further proved by the increased quasi-stationary concentrations of both HO(•) and SO(4)(•-) at the pH range of 8-10. The formation of HO(•) and SO(4)(•-) in the UV/PMS system was confirmed mainly from the cooperation of the photolysis of PMS, the decay of peroxomonosulfate radical (SO(5)(•-)) and the conversion of SO(4)(•-) to HO(•) by simulation and experimental results. Additionally, the apparent quantum yield for SO(4)(•-) in the UV/PMS system was calculated as 0.52 ± 0.01 at pH 7. The conclusions above as well as the general kinetic expressions given might provide some references for the UV/PMS applications.

[Effect of preozonation on disinfection by-products formation potential of algae cells and extracellular organic matter].

Water containing Oscillatoria agardii was cultured under controlled conditions and harvested in the late log growth phase. The objective was to determine: the contribution of algae cells and algae extracellular organic matter (EOM) to the disinfection by-products formation potential (DBPFP), and the effects of preozonation including ozone dosage and preozonation time on DBPFP of algae cells and EOM and mechanism of these effects. The results show that the main trihalomethanes from both Oscillatoria cells and EOM are chloroform and bromodichloromethane, and that the main haloacetic acids are dichloroacetic acid and trichloroacetic acid. HAAFP from algae cells and EOM themselves or after preozonation followed by coagulation is more than THMFP, which shows that more attention should be paid to the control of HAA in the treatment of algae containing water. DBPFP of EOM is reduced by preozonation, and DBPFP clearly decreases with time. Compared with traditional coagulation, the dosage of 0.975 mg/L preozonation with reaction time of 10 minntes followed by coagulation can decrease DBPFP of EOM by 31% and decrease HAAFP by 52.6%, but increase THMFP by 12.5% under this experiment's condition. This result shows the major reason preozonation can control the DBPFP of EOM is that it can control HAAFP effectively. At the same time, the DBPFP of algae cells is significantly increased by preozonation, and this increase is almost a linear function of preozonation time. It can be concluded that in the real water treatment case most of algae cells should be removed intact before preozonation to control the DBPFP.